Duplex ferritic austenitic stainless steel
11566309 · 2023-01-31
Assignee
Inventors
Cpc classification
C22C38/005
CHEMISTRY; METALLURGY
International classification
Abstract
Disclosed is a duplex ferritic austenitic stainless steel of 40-60 volume % ferrite and 40-60 volume % austenite, with improved cold workability and impact toughness. It contains less than 0.07% carbon (C), 0.1-2.0% silicon (Si), 3-5% manganese (Mn), 19-23% chromium (Cr), 1.1-1.9% nickel (Ni), 1.1-3.5% copper (Cu), 0.18-0.30% nitrogen (N), optionally molybdenum (Mo) and/or tungsten (W) according to the formula (Mo+½W)<1.0%. It optionally contains 0.001-0.005% boron (B), up to 0.03% of each of cerium (Ce) and/or calcium (Ca), with the balance being iron (Fe) and impurities where the chromium equivalent (Cr.sub.eq) and the nickel equivalent (Ni.sub.eq): 20<Cr.sub.eq<24.5 and Ni.sub.eq>10, where Cr.sub.eq=Cr+1.5Si+Mo+2Ti+0.5Nb Ni.sub.eq=Ni+0.5Mn+30(C+N)+0.5(Cu+Co).
Claims
1. A duplex ferritic austenitic stainless steel having 40-60 volume % ferrite and 40-60 volume % austenite, having undergone annealing at a temperature of 1050° C., having an impact strength of at least 27.5 J, wherein the steel consists essentially of in weight % less than 0.07% carbon (C), 0.1-2.0% silicon (Si), 3-5% manganese (Mn), 19-23% chromium (Cr), 1.1-1.9% nickel (Ni), 1.1-1.5% copper (Cu), 0.18-0.30% nitrogen (N), 0.1-1.0% molybdenum (Mo), greater than 0%-0.5% tungsten (W), molybdenum (Mo) and tungsten (W) in a total amount abides by the formula 0.1%<(Mo+½W)≤1.0%, up to 0.03% of each of cerium (Ce) and/or calcium (Ca), optionally 0.001-0.005% boron (B), balance being iron (Fe) and inevitable impurities in conditions for the ferrite formers and the austenite formers, wherein for a chromium equivalent (Cr.sub.eq) and a nickel equivalent (Ni.sub.eq):
20<Cr.sub.eq<24.5 and Ni.sub.eq>10, wherein
Cr.sub.eq=Cr+1.5Si+Mo+2Ti+0.5Nb;
Ni.sub.eq=Ni+0.5Mn+30(C+N)+0.5(Cu+Co), and wherein the duplex ferritic austenitic stainless steel has an average critical pitting temperature (CPT) of 14.5-17.7° C.
2. The duplex ferritic austenitic stainless steel according to claim 1, having 20-22 weight % chromium.
3. The duplex ferritic austenitic stainless steel according to claim 1, having 21-22 weight % chromium.
4. The duplex ferritic austenitic stainless steel according to claim 1, having 21.2-21.8 weight % chromium.
5. The duplex ferritic austenitic stainless steel according to claim 1, having 1.35-1.9 weight % nickel.
6. The duplex ferritic austenitic stainless steel according to claim 1, having 3.8-5.0 weight % manganese.
7. The duplex ferritic austenitic stainless steel according to claim 1, having 3.8-4.5 weight % manganese.
8. The duplex ferritic austenitic stainless steel according to claim 1, having 0.20-0.26 weight % nitrogen.
9. The duplex ferritic austenitic stainless steel according to claim 1, having 0.20-024 weight % nitrogen.
10. The duplex ferritic austenitic stainless steel according to claim 1, having 0.1-0.8 weight % molybdenum (Mo).
11. The duplex ferritic austenitic stainless steel according to claim 1, having 0.1-0.65 weight % molybdenum (Mo).
12. The duplex ferritic austenitic stainless steel according to claim 1, having 0.15-1.0 weight % molybdenum (Mo).
13. The duplex ferritic austenitic stainless steel according to claim 1, having 0.15-0.8 weight % molybdenum (Mo).
14. The duplex ferritic austenitic stainless steel according to claim 1, having 0.15-0.65 weight % molybdenum (Mo).
15. The duplex ferritic austenitic stainless steel according to claim 1, having 0.15-0.54 weight % molybdenum (Mo).
Description
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
(1) The test results of the ferritic austenitic stainless steels of the invention are illustrated in more details in the following drawings, where
(2)
(3)
(4)
DETAILED DESCRIPTION OF THE INVENTION
(5) The effect of copper to the cold workability properties was tested using for each alloy the 30 kg melts received from a vacuum furnace. Before mechanical testing, the alloys were forged to a final thickness of 50 mm. For all melts the duplex ferritic austenitic stainless steel marketed under the trademark LDX 2101® was used as the base material with varying additions of copper. The chemical compositions of the alloys to be tested are described in the table 1, which also contains the chemical composition for the respective melt of the steel marketed under the trademark LDX 2101®:
(6) TABLE-US-00001 TABLE 1 Fe + Alloy C Si Mn Cr Ni Cu N impurities LDX 0.021 0.76 4.83 21.34 1.58 0.40 0.240 Balance 2101 ® 0.75% Cu 0.014 0.59 4.09 21.56 1.62 0.74 0.186 Balance 0.85% Cu* 0.028 0.83 3.81 21.08 1.50 0.86 0.25 Balance 1.0% Cu 0.015 0.60 4.14 21.22 1.93 1.01 0.200 Balance 1.1% Cu* 0.013 0.88 4.42 21.18 1.36 1.12 0.22 Balance 1.5% Cu 0.015 0.55 4.15 21.33 1.6 1.50 0.188 Balance 2.5% Cu* 0.028 0.79 4.04 21.17 1.36 2.55 0.19 Balance 3.5% Cu* 0.012 0.80 3.82 20.87 1.38 3.57 0.21 Balance Chemical compositions; *200 g small scale melt
(7) The microstructure investigations were performed primarily to check the ferrite content. This is, because copper is an austenite stabiliser and it was expected that the austenite content was increased with the additions of copper. When maintaining the ferrite content at least 45 volume %, the manganese content, as an austenite stabilizer, was reduced to approximately at the range of 3-5%. It was also considered necessary for the copper to be fully dissolved within the ferrite phase since copper particles or copper rich phases can be detrimental to the pitting corrosion resistance.
(8) The microstructures of the samples were revealed by etching in Behara II solution after annealing at the temperature of 1050 and/or 1150° C. The annealing was done by solution annealing. The microstructure of the 0.85% Cu alloy is essentially the same as the reference alloy. At the copper levels of 1.1% Cu and higher the ferrite phase content becomes successively low. The secondary austenite phase forms readily with the additions of 2.5% Cu and copper particles are present in the ferrite phase when annealed at the temperature of 1050° C., but can be dissolved when annealed at the temperature of 1150° C. as the ferrite content increases. The alloy with 3.5% Cu has copper particles in the ferrite phase even when annealed in the temperature of 1150° C.
(9) The ferrite contents for the annealed samples at the annealing temperatures (T) 1050° C. and 1150° C. were measured using image analysis, and the results are presented in the table 2:
(10) TABLE-US-00002 TABLE 2 Ferrite contents Alloy As-forged (%) T 1050° C. (%) T 1150° C. (%) LDX 2101 ® 62.5 54.1 0.75% Cu 66.2 60.2 0.85% Cu 53.1 1.0% Cu 61.5 55.4 1.1% Cu 44.2 50.0 1.5% Cu 62.4 52.7 2.5% Cu 39.0 35.5 3.5% Cu 39.3 42.0
(11) From the results of the table 2 it is noticed that up to a copper content 1.5% the ferrite content is fine, but at the levels greater than this the ferrite content is too low even when annealed at the higher temperature. Typically, increasing the annealing temperature the ferrite content increases by 5-7 volume % as it is the case for the 1.1% Cu and 3.5% Cu alloys. The ferrite content for the 2.5% Cu is the same at both the annealing temperatures. This is probably due to copper being fully dissolved into the ferrite phase at the higher (1150° C.) temperature resulting in the formation of secondary austenite phase counteracting the increase in the ferrite phase.
(12) For the alloys 0.75% Cu, 1.0% Cu and 1.5% Cu the microstructure was determined in the as-forged condition, in which case the ferrite content was between 61-66% for all those alloys. After annealing at the temperature of 1050° C. there was a decrease in the ferrite content by approximately 6-8% for all alloys. From the image analysis it was observed that the decrease in the ferrite content is mostly due to the presence of secondary austenite phase that becomes more apparent as the copper content was increased. In the 1.5% Cu alloy a great deal of the austenite phase exists between the ferrite grains.
(13) The critical pitting temperatures (CPT) were determined for the alloys annealed at the temperature of 1050° C. according to the ASTM G150 test with 1.0 M NaCl. For each alloy the test was done two times (CPT1 and CPT2). The results of these tests are presented in the table 3:
(14) TABLE-US-00003 TABLE 3 Critical pitting temperatures (CPT) CPT1 ° C. CPT2 ° C. CPT Average ° C. LDX 2101 ® 11.4 9.7 10.6 1.1% Cu 15.7 13.4 14.5 3.5% Cu 16.6 18.9 17.7
(15) The results in the table 3 show that in this environment a positive effect of copper on the CPT is given. The CPT is actually highest for the 3.5% alloy despite the presence of copper particles in the microstructure. Surprisingly, this contradicts somewhat the hypothesis that copper particles are detrimental to the pitting resistance.
(16) The testing for cold heading as a part for cold workability was performed on samples in the as-forged and annealed (1050° C.) conditions in order to determine that the duplex ferritic austenitic stainless steel of the invention has better properties when compared with the reference material LDX 2101®. The materials were machined to cylindrical samples with the dimensions of 12 mm×8 mm for compressing the samples at high rates of 200-400 mm/s. Samples were evaluated by noting cracking (failed components) or crack free (passed components).
(17) In this testing method cracking only occurred when the sample was compressed with maximum compression to an actual final thickness of approximately 3 millimeter regardless of the compressing speed. Cracking was slightly more severe under compression at higher speeds.
(18) The cold heading test results are presented in the table 4, where the samples are in the as-forged condition except when annealed at the temperature of 1050° C. the column “Annealed” is provided with the term “Yes”:
(19) TABLE-US-00004 TABLE 4 Results of mechanical testing Annealed Actual final Rate (T = 1050° C.) thickness (mm) (mm/s) Result LDX 2101 ® No 2.7 200 Small crack LDX 2101 ® Yes 2.5 200 No cracks LDX 2101 ® No 2.5 200 Large crack LDX 2101 ® Yes 2.5 200 Small cracks LDX 2101 ® No 2.5 300 Crack LDX 2101 ® Yes 2.5 300 Cracks LDX 2101 ® Yes 2.4 300 Small cracks LDX 2101 ® No 2.4 400 Crack LDX 2101 ® Yes 2.5 400 No cracks LDX 2101 ® No 2.4 400 Crack LDX 2101 ® Yes 2.5 400 Large cracks 0.75% Cu No 2.4 200 Crack 0.75% Cu Yes 2.3 200 Crack 0.75% Cu No 2.5 200 Crack 0.75% Cu Yes 2.4 200 No cracks 0.75% Cu No 2.5 300 Small crack 0.75% Cu Yes 2.4 300 Crack 0.75% Cu No 2.4 300 Large crack 0.75% Cu Yes 2.4 300 Large cracks 0.75% Cu No 2.6 400 Crack 0.75% Cu Yes 2.3 400 Large cracks 0.75% Cu No 2.6 400 Crack 0.75% Cu Yes 2.3 400 Large cracks 1.0% Cu No 2.7 200 No cracks 1.0% Cu Yes 2.7 200 Cracks 1.0% Cu No 2.6 300 Small cracks 1.0% Cu Yes 2.4 200 Cracks 1.0% Cu No 2.7 300 Small cracks 1.0% Cu Yes 2.6 300 No cracks 1.0% Cu Yes 2.5 300 Small cracks 1.0% Cu No 2.5 400 No cracks 1.0% Cu Yes 2.6 400 No cracks 1.0% Cu Yes 2.4 400 Small cracks 1.5% Cu No 2.4 200 No cracks 1.5% Cu No 3.1 200 No cracks 1.5% Cu No 2.5 200 No cracks 1.5% Cu yes 3.1 200 No cracks 1.5% Cu yes 2.5 200 No cracks 1.5% Cu yes 2.5 200 Small cracks 1.5% Cu No 2.5 300 No cracks 1.5% Cu No 2.5 300 No cracks 1.5% Cu yes 2.4 300 No cracks 1.5% Cu yes 2.5 300 Small crack 1.5% Cu yes 2.5 300 No cracks 1.5% Cu No 2.4 400 No cracks 1.5% Cu No 2.4 400 Cracks 1.5% Cu Yes 2.5 400 Crack 1.5% Cu Yes 2.4 400 Small crack 1.5% Cu Yes 2.5 400 No Cracks
(20) The results in the table 4 show that in tests on the forged material all the samples for LDX 2101° and 0.75% Cu failed because of cracking, whereas the success rate increased as the copper content is increased. All but one of 1.5% Cu samples passed the test in the as-forged condition. After annealing at the temperature of 1050° C., the alloys with up to 1.0% Cu show similar results with approximately one third of the samples passing the test For the 1.5% Cu alloy more than half of the tested components passed the test indicating the positive effect of copper.
(21) The cold heading test results are also shown in the
(22) The ferritic austenitic stainless steels of the invention were further tested by measuring the impact strength of the steels in order to have information of the impact toughness of the steels. The measurements were made both in an as-forged condition and after annealing at the temperature of 1050° C. In the table 5, the samples are in the as-forged condition except when annealed at the temperature of 1050° C. the column “Annealed” is provided with the term “Yes”. Both the table 5 and the
(23) TABLE-US-00005 TABLE 5 Results of impact testing Annealed Impact strength (T = 1050° C.) J LDX 2101 ® No 14.5 LDX 2101 ® Yes 20.5 0.75% Cu No 10.5 0.75% Cu Yes 14.5 1.0% Cu No 17.0 1.0% Cu Yes 27.5 1.5% Cu No 28.5 1.5% Cu Yes 36.0
(24) The results in the table 5 and in the
(25) The duplex ferritic austenitic steel manufactured in accordance with the invention can be produced as castings, ingots, slabs, blooms, billets and flat products such as plates, sheets, strips, coils, and long products such as bars, rods, wires, profiles and shapes, seamless and welded tubes and/or pipes. Further, additional products such as metallic powder, formed shapes and profiles can be produced.
SEQUENCE LISTING
(26) Not Applicable.